(2) Management and condition of the site prior to the introduction
(3.1) The introduction of Plebejus argus.
(3.2) Comments on the introduction and observations of egg-laying.
(4) Future management
(5) Future monitoring
An introduction of the Silver-studded Blue butterfly (Plebejus argus) has been carried out at Thurstaston Common (SJ 248846) using stock from Prees Heath, Shropshire (SJ 565375) during June and July of 1994. In total 54 female and 18 male butterflies were released in four batches between 28/6/94 and 8/7/94. The weather was variable over this period, but the butterflies seemed to settle to the site well and seven examples of egg-Iaying were recorded. This report outlines the condition of the site prior to introduction and the details of the introduction itself. Further progress reports will follow in subsequent years.
The known habitat requirements of the Silver-studded Blue butterfly on heathland sites are outlined below:
(i) A sheltered, sunny site preferably south facing.
(ii) Areas of pioneer heath with a mixture of bare ground and young shoots of heather (particularly Calluna vulgaris and Erica cinerea) or gorse (Ulex species), to act as a larval foodplant and provide a hot, dry microclimate for larval survival.
(iii) A prevalence of ants particularly Lasius species and particularly in areas of pioneer heath.
(iv) Nectar source plants for adults (e.g. Erica cinerea).
(v) Roosting sites for adults (e.g. long grass, mature heather and gorse).
A sketch map of the Thurstaston Common site, lying between the Heatherlands restaurant and The Cottage Loaf public house, at SJ 248846 is attached. The original site, outlined in reference (1), of approximately 6 acres has been extended during winter 93/94 to include areas 1, 2 and 3 (see sketch map) and now comprises some 8 acres of heathland. The site is sheltered in all directions by birch scrub and gorse and is flat apart from two south facing banks, both of which contain an abundance of Erica cinerea. The heather forms of these slopes are of varied structure, but contain mainly older plants (10+ years). The areas marked A,B and C were subjected to burning in 1989 in order to eradicate heather beetle and have been slow to recover. Much of the Calluna in these areas is still only 5-10cm in height and significant areas remain bare. Birch scrub removal was carried out on A, B and C during winter 92/93 and 93/94. This scrub is under control although the richer soils in the former scrub have been invaded by Wavy Hair-grass (Deschampsia flexuosa). In winter 92/93 approximately 200 flat sandstone rocks were placed in bare areas on A, B and C in an attempt to provide suitable nesting sites for the black ant, Lasius niger. In the first summer 25% of the stones were occupied and this figure rose to 60% in the summer of 94. We feel that this has been a worthwhile task and has increased the ant densities on A, B and C. Area D was burnt in a late summer fire in 1991 and regrowth has again been slow, but more uniform than on A, B and C. The majority of Calluna and Erica on area D is less than 10cm in height. It was agreed that areas A, B, C and D contained a good mosaic of heather forms much of which should still be suitable for egg-laying and larval survival of Plebejus argus. Thus these areas were not subjected to any further burning/flailing before the introduction.
Prior to winter 93/94 areas 1, 2 and 3 consisted of mature heather (mainly Calluna), gorse and birch scrub. Last winter the birch scrub was removed from 1, 2 and 3 and the area 1 selected for burning. A fire break of some 2 metres width was produced by flailing and raking the heather on the perimeter of 1. Area 1 was burnt using a gas lance in February 1994. By June 1994 the burnt area was showing healthy signs of regeneration, whilst the flailed fire break appeared slower to respond. Of the areas 1, 2 and 3 only 1 was deemed to contain suitable pioneer heath for larval survival. The areas 2 and 3 would be left mature until area 1 was becoming unsuitable, then a rotational burning regime would be introduced.
During the flight period of the Silver-studded Blue butterfly at Prees Heath, Shropshire (SJ 565 375) in 1994 it was not possible to predict an extended spell of hot, sunny weather. Numbers of adults at Prees Heath were high and so it was decided to carry out the introduction over a number of days, bringing batches of adults to Thurstaston and hoping for settled weather in the following days. This was preferred to a single introduction which would depend too heavily on good weather over a short time period. Four separate batches of Plebejus argus adults were removed from Prees Heath. Each individual adult was carried in a small, dark plastic tube and placed in a cold box for the approximately 1 hour journey to Thurstaston. There appeared to be no casualties from this transfer, all adults reviving quickly from the journey and flying from the opened tubes. The timetable of releases was as follows:
Release - 10 females and 5 males released on area A (see sketch map) at 1430 hours. Conditions - sunny but very breezy.
Observation of previous release - A total of 5 females and 4 males found on areas A and B from previous release.
Release - 10 females and 5 males released on area A at 1330 hours. Conditions - overcast with hazy sunshine, light breeze.
Observation of previous releases - A total of 7 females and 3 males found on areas A and B from previous releases.
Release - 18 females and 3 males released on area 1 at 1400 hours. Conditions - overcast and humid, no breeze.
Observation of previous releases - A total of 5 females and 2 males found on areas A and B. A total of 9 females and 2 males found on area 1.
Observation of previous releases - A total of 2 females and 2 males found on areas A and B. A total of 2 males and 2 females found on area 1.
Release - 16 females and 5 males released on area D at 1600 hours. Conditions - cloudy and cool.
Observation of previous releases - A total of 1 female and 2 males on areas A and B. A total of 2 females and 1 male on area 1. A total of 6 females and 3 males on area D.
Total of Plebejus argus released = 54 female and 18 male
Despite very mixed weather during the introduction period at Thurstaston we are quite hopeful that Plebejus argus will take to this new site. Introduced adults appeared to be happy with the site and settled in well. Most moved only a short distance (less than 50 metres) from the introduction spot, but on occasion some were seen to travel over 50 metres in one flight. Individuals appeared to survive a number of days and it was promising to observe some butterflies on Friday 8/7/94, five days after a release and following heavy rainfall on Wednesday 6/7/94. Adults appeared to be content to use Erica cinerea as a nectar source and found suitable roosting sites in mature heather, gorse and in long grass. Roosting sites appeared to change depending on prevailing wind and relative shelter.
The sunny conditions required for egg-laying were infrequent during our observation time, but seven examples were noted all in the Calluna rich areas of A and B. Egg-laying was not observed on areas C, D and 1 but females were present in these areas and with the infrequency of suitable conditions during observation it is not feasible to assume that these areas were unsuitable. All examples of egg-laying were on Calluna vulgaris (though this is the major species in areas A and B). In one case the egg was placed on dead Calluna close to fresh shoots, all others were on fresh shoots. One egg was placed 14cm from the ground the rest at 10cm or below, the lowest being at 3cm. The edges of heather plants did not particularly seem to be chosen, most being laid on areas with a low cover of heather and bare ground beneath. Grassy areas were not chosen for egg-laying. In all cases but one ant activity (Lasius) was observed very close to the egg-laying position.
Some sensitive scrub removal will be carried out during winter 1994/95. More sandstone will be situated on the site, particularly area 1, to promote increased ant concentrations. Areas 1, A, B, C and D will be carefully monitored in future years as to their suitability for egg-laying and larval survival. Areas 2 and 3 will be burnt in rotation with 1 at a suitable time. A second rotation will be instigated on areas A, B, C and D again at a suitable time.
The Thurstaston site will be monitored each year and results reported.
Hinde, D.C. A proposal for the introduction of the Silver-studded Blue butterfly (Plebejus argus) at Thurstaston Common, Wirral.
Back to Silver-studded Blue